Rear End Ratios Explained

What's rear end gear ratio?
How does rear en ratio affect your vehicle's performance?
How to calculate your rear end, gear, rpm ratio?

Rear end Ratio is the ratio of how many times the drive shaft turns to how many times the axle turns. This is set up by how many teeth are on rear end's ring and pinion gears.

A higher ration number will give you more torque (power) at the rear wheels, better acceleration ( more low end pulling power), but lower top speed and less fuel mileage.

A Lower ration number will give you lower acceleration but higher RPM's, more high speed (RPM's) and also more fuel consumption, the higher RPM's the poorer your gas mileage is.

A bit confusing but... Higher ratio is referred to as lower gearing (geared down) and lower ratio is referred to as higher gearing. Power is transmitted from the transmission to rear end 1:1 in high gear.

For example, If your vehicle is equipped with 3.55 gear, the ratio is 3.55 to 1. Meaning that for every
3.55 times that the driveshaft turns, the rear axle turns one time and you will have a torque advantage of  3.55x. So for every 10 turns of the (rear) wheels, your drive shaft will turn 35.5 times and the engine will only turn 35.5 times in high gear.

If your vehicle is running a 3.73 rear end gear set up...  the ratio is 3.75 to 1. This means that for every 3.73 turns of the driveshaft, the rear axle will be turned one time and you will have a torque advantage of 3.73x. In this case  every 10 turns of the (rear) wheels, your drive shaft will turn 37.5 times and the engine has to turn 37.5 times in high gear.  



Another example to make it clear... when your vehicle is running a 4.10 rear end gear set up...  the ratio is 4.10 to 1. This means that for every 4.1 turns of the driveshaft, the rear axle will be turned one time and you will have a torque advantage of 4.1x. In this case  every 10 turns of the (rear) wheels, your drive shaft will turn 41 times and the engine has to turn 41 times in high gear.  

Keep in mind... that the gearing is relative to the size tire your are using. As mentioned above... first two examples, the 3.55 will give better gas mileage over the 3.73 for a given tire size range. Get above this range, say for 3.55 gears, and the performance will suffer, as well as putting additional strain on the tranny, gears, etc.

Didn't mean to complicate this article, but you can't talk gearing without mentioning tire size, and effecting the RPM's.

The "higher" gearing number (ex. 4.10 vs. 3.75) will allow for larger tires that will come close to offsetting the RPM diff.

RPM - Rear End Ratio Chart.
Following chart can help you decide which rear end ratio is the best for you / your vehicle.
Listed are the expected RPM's at 60 MPH with a given rear end ratio and tire height.
The Chart applies to all Ford, both Automatic & Manual transmissions.
NON OVERDRIVE... such as: C-4 three speed automatic, C-6 three speed automatic, FMX three speed automatic, Top-loader three speed manual and Top-loader four speed, manual

Running an overdrive transmission, the RPM's will be about 32% less.

A good tool also is... a gearing calculator, knowing the size tires you want to run and how you will use the car 100% highway, 50% Highway 50% Racing or 100% racing... you can either pick the Gearing or Rpm and calculate the other (Rpm or Gearing).

The Formula to calculate RPMs is...
RPM = MPH X Trans Ratio (in high gear) X Rear End Ratio X 336 : Tire Height

Below are three useful ratio / gearing / rpm calculator Links,

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